Images are typically stored in a memory representing tone values for each pixel of the original image. For a black and white image, the stored pixels represent the gray scale value corresponding to each pixel. For a color image, each color plane is stored as an array of pixels each representing the tone value for each pixel of the image in each respective color plane. For example, if each of the pixels of a black and white image is represented by a 8 bit digital word, then the tone value for a given image pixel may be one of 256 values between the black level and the white level.
Continuous tone images do not print well on most printing devices where typically the absence or presence of the ink on the paper is used to represent the printed image. In order to represent halftones (shades between the presence or absence of the printed ink), the original image is screened to produce a pattern, such a variable size dots which appear to the human eye as a halftone image.
Screening to produce halftone images is well known. The screen consists of an array of dots, or halftone cells, each of which represents one section of continuous tone in the original image as a single dot of variable size and shape. A halftone cell, in turn, consists of an array of smaller screen cells each having individual values against which the input pixels derived from the original image will be compared. The screen is usually stored as a fairly small pattern that repeats itself. If the value of the image pixel is greater than corresponding value of the screen cell, a mark is generated by the marking engine, whereas if the value of the image pixel is less or equal to the screen cell value, then no mark is generated by the marking engine, or vice versa.
In the prior art, techniques for mechanical and electronic screening of images, using a great variety of specific halftone screening patterns, and at various screening angles, are well known to those skilled in the art, In general, the halftone screen is much finer than the original image. That is, in order to represent the halftone by a variable shaped dot of solid color, the halftone cell typically has more screen cells than there are original image pixels. The output device and the screen typically have the same spatial resolution, but the original image usually must be enlarged in size. For example, one scan line on the output device may be 8000 pixels, but only 2000 pixels of the original image were scanned. Therefore, an enlargement ratio of 4 must be used. In general, it is necessary to enlarge the original image by an enlargement factor so that the enlarged image has the same number of pixels as the screen has screen cells. Also, in cases where only a portion of the input image is to be printed, the portion to be printed must be enlarged to fit the screen and final image size. Usually, image enlargement and screening are performed in the same process. The enlargement ratio is almost always greater than one. If the enlargement ratio is less than one, the reduction is performed elsewhere, prior to screening.
To enlarge an image, pixels are repeated. For example, to enlarge an image by a factor of 4, each pixel is repeated 4 times in each of the horizontal and vertical directions. To enlarge an image by a factor of 2.5, each pixel is repeated two times for one half the time, and three times for the other half of the time, in order to average 2.5 times. The enlargement method is typically accomplished by adding a number equal to the reciprocal of the desired enlargement ratio to a register. The previous pixel value is repeated until the register overflows, after which the next pixel is repeated until the register overflows again. Screening (comparing pixels) is performed at the same time as enlargement (repeating pixels). After each register addition of the reciprocal and test for overflow, the resulting input pixel is compared to the appropriate screen cell to generate a screened image.
A key performance measure in screening an image is speed. A fine screen results in a high quality image, but the more cells in the screen, the longer it will take to screen an image. Also, for color separations, four screens, one for each of yellow, magenta, cyan and black are required. Therefore many image screening apparatus typically implement the screening steps in hardware, which generally is faster than a corresponding application of the same methods in software. The present invention makes implementation in software practical and faster than prior art methods.